LeetCode 374. Guess Number Higher or Lower

We are playing the Guess Game. The game is as follows:

I pick a number from 1 to n. You have to guess which number I picked.

Every time you guess wrong, I'll tell you whether the number is higher or lower.

You call a pre-defined API guess(int num) which returns 3 possible results (-1, 1, or 0):

-1 : My number is lower
 1 : My number is higher
 0 : Congrats! You got it!

Example:

n = 10, I pick 6.

Return 6.

  一个二分查找

// Forward declaration of guess API.
// @param num, your guess
// @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
int guess(int num);

class Solution {
public:
    int guessNumber(int n) {
        long long start=1,end=n;
        long long mid=(start+end+1)/2;
        if(0==guess(start))return start;
        while(guess(mid))
        {
            if(-1==guess(mid))
            {
                end=mid-1;
                mid=(start+end+1)/2;
            }
            if(1==guess(mid))
            {
                start=mid+1;
                if(0==guess(start))return start;
                mid=(start+end+1)/2;
            }
        }
        return mid;
    }
};